add nor, xnor chips

projects/custom/Nor.hdl

@@ -0,0 +1,22 @@
+// This file is part of www.nand2tetris.org
+// and the book "The Elements of Computing Systems"
+// by Nisan and Schocken, MIT Press.
+// File name: projects/custom/Nor.hdl
+
+ /**
+ * Nor gate:
+ * out = 1 if (a == 0 or b == 0)
+ *       0 otherwise
+ */
+
+CHIP Nor {
+    IN a, b;
+    OUT out;
+
+    PARTS:
+    // Put your code here:
+    Nand(a=a, b=a, out=outx);
+    Nand(a=b, b=b, out=outy);
+    Nand(a=outx, b=outy, out=outz);
+    Nand(a=outz, b=outz, out=out);
+}

projects/custom/Xnor.hdl

@@ -0,0 +1,23 @@
+// This file is part of www.nand2tetris.org
+// and the book "The Elements of Computing Systems"
+// by Nisan and Schocken, MIT Press.
+// File name: projects/custom/Xnor.hdl
+
+ /**
+ * Or gate:
+ * out = (a == b)
+ *       0 otherwise
+ */
+
+CHIP Xnor {
+    IN a, b;
+    OUT out;
+
+    PARTS:
+    // Put your code here:
+    Nand(a=a, b=b, out=outw);
+    Nand(a=a, b=outw, out=outx);
+    Nand(a=b, b=outw, out=outy);
+    Nand(a=outx, b=outy, out=outz);
+    Nand(a=outz, b=outz, out=out);
+}